In Release 8 of the Long Term Evolution (LTE) standards, as promulgated by the THIRD GENERATION PARTNERSHIP PROJECT (3GPP), a mobile terminal or other User Equipment (UE) is assigned data on a time-frequency basis. A given allocation of particular radiofrequency sub-carriers for a given interval of time is referred to as a Resource Block (RB), and resource allocations (uplink or downlink) generally are made on an ongoing, scheduled basis.
Accordingly, an LTE base station, referred to as an “eNodeB,” transmits control information to mobile terminals that, among other things, identify scheduled resource allocations. In particular, on the Orthogonal Frequency Division Multiplex (OFDM) downlink, the eNodeB transmits Downlink Control Information (DCI) to targeted ones in a plurality of mobile terminals on a Physical Downlink Control Channel (PDCCH), and transmits user data to targeted ones in the plurality of mobile terminals on an associated Physical Downlink Shared Channel (PDSCH). An example PDCCH/PDSCH sub-frame is shown in FIG. 1.
The PDCCH and the associated PDSCH are transmitted within repeating sub-frames of the OFDM signal, e.g., the first N symbol times of each given sub-frame are allocated as the PDCCH, and the remainder of the sub-frames are allocated as the PDSCH. In each sub-frame, each mobile terminal blindly decodes the PDCCH, looking for a DCI message targeted to it. Terminal-specific Medium Access Control (MAC) layer identifiers are used to indicate the targets of the transmitted DCI messages. There are different types of DCI messages, but they include downlink resource assignment messages. If a mobile terminal receives a downlink resource assignment message in a given sub-frame, it uses that information to identify the particular time-frequency resource allocations used in the PDSCH of that sub-frame to carry user data targeted to the mobile terminal.
Consequently, failure by the mobile terminal to correctly decode the DCI message leads to data reception failures, e.g., the mobile terminal will not detect a targeted DCI message and therefore miss the corresponding transmission of user data on the PDSCH, or, while not likely because of CRC protection, it may incorrectly identify the particular PDSCH resource allocation for its user data, and attempt to decode the wrong data. Depending upon the type of DCI message involved, DCI decoding failures have other consequences, such as missed or incorrectly scheduled uplink transmissions, power control interruptions or misbehavior, etc. One may refer to the Technical Specification TS36.212 for comprehensive DCI details, but it may be helpful to identify selected details here.
For example, information about location, modulation and the eNodeB's transmission schemes, etc., is included in the PDCCH of each sub-frame. For a given system bandwidth, the number of OFDM symbols to be used for this control signaling mainly depends on the number of mobile terminals that are to be scheduled in the current sub-frame. Each DCI (message) contains information identifying how to unambiguously decode its scheduled assignments for a particular mobile terminal.
In the current standards, there is an upper limit on how many bits can be used to encode the DCI for a given mobile terminal. Of course, the number of total bits available for transmitting a DCI places a defined upper limit on the amount of error protection coding available. That is, the DCI bit totals define a maximum coding gain available. More particularly, a DCI can be coded into up to 8 Channel Control Elements (CCEs), where a CCE is defined as 36 Resource Elements (REs), or QPSK symbols, which is equivalent to 72 bits. With this arrangement, an example of which appears in FIG. 2, the lowest coding rate becomes X/576, where X is the number of un-coded bits—including a 16-bit CRC—for the DCI. This code rate mainly determines the decoding performance achievable at the targeted mobile terminal, and thus determines that terminal's ability to reliably decode scheduled assignments.
For a small system bandwidth, where a low number of RBs are available within a given defined OFDM frequency band, the number of mobile terminals and the corresponding individual code rates that can be scheduled is limited by the number of CCEs available. In certain reception conditions, and for certain DCI coding rates, reception performance, e.g., Block Error Rate (BLER) performance, for DCI decoding at a given mobile terminal may be unacceptable. For example, assuming an LTE OFDM bandwidth of 1.4 MHz, eNodeB transmission from two antennas, and a DCI coding rate of ⅙, relatively high BLER is experienced by a mobile terminal operating under a typical RAN4 test scenario.